1. Field of Invention
The invention relates to an apparatus for rapid automated on-line continuous measurement of chemical composition of ambient aerosol particles. This invention also relates to a method of analyzing the chemical composition of ambient aerosol particles.
2. Description of Related Art
There have been many devices built and many processes developed to understand the sources, atmospheric transformation, fate and health effects of ambient aerosols. All require knowledge of particle chemical composition. Most processes using these devices are quantitative composition measurements, typically performed off-line on particles collected onto substrates by filtration or inertial impaction. Samples collected on the substrates are then manually extracted and analyzed. For example, measurements of ionic aerosol components involve collection on denuder-filter pack assemblies, extraction of the collected aerosol into water, and analysis of the extract for various ionic species using an ion chromatography (IC) technique. Unfortunately, although widely used, this approach has many drawbacks. Additionally, depending on flow rates and ambient concentrations, the sampling intervals are long, typically hours to days. Faster measurements are possible but often impractical due to the labor involved in filter preparation and sample extraction. This is particularly true in long-term air quality monitoring programs. Since the turn-around time for processed results also tends to be long, immediate insight and interpretation of these results are generally not available for in-the-field adjustment and modifications of experiment sampling strategies and protocols.
In addition, off-line techniques are also prone to potential sampling artifacts, particularly for volatile aerosol components. Artifacts that lead to measurement errors are due to particle/gas, particle/particle, gas/substrate, and particle/substrate interactions (Chow, 1995). These interactions occur because particles are removed from the gas, concentrated on the substrate, and then exposed to different conditions for extended periods during sampling and storage. During sampling, volatile chemical components can be adsorbed or lost as result of changes in temperature, relative humidity, and ambient particle and gas composition. Pressure drops within the sampler can also contribute to volatility losses. These artifacts have led to complicated filter pack sampling systems using multiple filters of various types to capture volatile aerosol components. Artifacts may also be introduced in the preparation and extraction of filters. Combined, these processes can lead to significant uncertainties, particularly when measuring mass concentrations of volatile or easily contaminated aerosol chemical components, such as nitrate, ammonium, and semi-volatile organic species.
Advanced instruments for real-time size-resolved measurements of particle chemical composition involving mass spectrometers have been developed (Carson et al., 1995; Hinz et al., 1994; Jayne et al., 1998; Marijinissen et al., 1988; McKeown et al., 1991; Prather et al., 1994; Reents et al., 1995). These techniques provide important insights into particle composition at single particle resolution. Unfortunately, they tend to be complex and costly, and the measurements generally do not give quantitative information on particle composition.
Other approaches involving automated bulk composition measurements have been developed. These approaches provide faster measurements and minimize some of the sampling artifacts associated with the off-line techniques. Although they do not provide size-resolved information as do mass spectrometer-based instruments, these approaches are quantitative. One common approach is to convert the aerosol particles to a vapor and measure selected evolved gases. For example, Turpin et al. (1990) developed a technique for carbonaceous aerosols by measuring the quantity of carbon dioxide produced when a loaded filter is heated to various temperatures. Stolzenburg and Hering (1999) developed an instrument that collects particles by impaction and measures various evolved gases when the deposited aerosol is flash vaporized. This approach has been successfully used to measure nitrate, and also shows promise for sulfate and carbonaceous aerosol components.
Other devices have been developed that bypass the filter or impactor sampling used in the off-line approaches for measurement of aerosol ionic species. In this case, the same analytical technique is employed, except the particles are collected directly into a liquid for automatic analysis by ion chromatography. Techniques for capturing the particles vary. Automated systems have been developed that collect particles onto a filter that is periodically washed (Buhr et al., 1995), or particles are directly impacted into a flowing liquid (Karlsson et al., 1997). In another approach, ambient particles are first grown to large water droplets by mixing with air saturated with water vapor. The large droplets are then captured onto surfaces by various inertial techniques, and combined with condensed water vapor, produce the liquid stream for analysis. A variety of instruments have been developed using this approach (Ito et al., 1998; Khlystov et al., 1995; Liu and Dasgupta, 1996; Oms et al., 1997; Poruthoor and Dasgupta, 1998; Simon and Dasgupta, 1995; Zellweger et al., 1999). Drawbacks associated with these techniques include aerosol losses, greater complexity due to the need for sample pre-concentration, and slow time response due to the time needed to drain large wetted areas.
What is needed, therefore, is an instrument designed specifically for rapid measurement of the chemical components of ambient aerosols.